The present invention relates to a new and improved construction of a sensor which makes use of a non-reciprocal optical effect.
In its more specific aspects, the present invention particularly relates to a new and improved construction of a sensor which makes use of a non-reciprocal optical effect and which comprises a passive ring resonator defining a closed light path, light source means, coupler means for coupling light from said light source means into said passive ring resonator with a first direction of propagation and a second direction of propagation opposite thereto and for coupling light out of said passive ring resonator, and detector means exposed to light coupled out by said coupler means.
An example of a non-reciprocal, optical effect is the Sagnac effect. A light path is defined by mirrors or light conductors which surrounds an area. In the state of rest, the optical path lengths of this light path for the light travelling clockwise and the light travelling counter-clockwise are identical. If, however this assembly makes a rotary movement, the optical path lengths for the light travelling clockwise and the light travelling counter-clockwise becomes different. The difference of the optical path lengths is proportional to the angular rate and to the area surrounded by the optical path. Angular rate sensors can be designed making use of this Sagnac effect.
Various types of sensors based on the Sagnac effect are known.
Glass fiber rate sensors comprise a glass fiber which is wound to form a coil of many turns around an area. The glass fiber has two ends. Light from a laser is directed by means of a beam splitter to both one end and the other end of the glass fiber. The light beam directed onto one end travels through the coil with a first direction of propagation, for example clockwise. The light directed onto the other end of the glass fiber travels through the coil with a second direction of propagation, thus, for example, counter-clockwise. The counter-clockwise light beam emerges at said one end. The clockwise light beam emerges at said other end. The two emerging light beams are superimposed and caused to produce interferences. The interfering light beams are directed onto a detector. The interference fringes are shifted, if the sensor is subjected to an angular rate. This is observed as detector output signal; see the publication by Holzapfel, entitled "Optische Kreiselsensoren", VDI-Bericht No. 509, 1984.
In order to achieve high sensitivity of such a glass fiber angular rate sensor, a long fiber in a coil having many turns is required. This, in turn, requires high quality fibers, in order to avoid inadmissibly strong attenuation. Such fibers are expensive.
German Published patent applications Ser. No. 3,805,904; 3,805,905; 3,912,005 and 3,919,060 show glass fiber angular rate sensors with six-port couplers.
Furthermore, sensors with ring lasers are known. A ring laser has a ring resonator. Such a ring resonator establishes a closed optical path, which may, for example, be defined by three plane mirrors. This optical path is established within a vessel, in which a gas discharge path is provided. The gas discharge path in the ring resonator acts as an optical amplifier. With sufficient gain of the gas discharge, the system will operate as a ring laser. A fraction of the generated laser beam is coupled out by means of a partially transparent mirror. With such ring lasers, it is known to cause interferences of the light beams coupled out. The interference pattern is detected ba two detectors located side by side. In the state of rest, the pattern of interference fringes is also at rest. When an angular rate is sensed, the optical path lengths of the ring resonator for the light beam travelling clockwise and the light beam travelling counterclockwise will be changed in opposite sense. Thereby the ring laser operates at different frequencies clockwise and counterclockwise. The interference fringes start to move over the detectors. The two detectors permit determination of a direction. Signals at the beat frequency are generated by the detectors. This beat frequency is proportional to the angular rate. Summing-up of the pulses generated by the interference fringes provides the angle of rotation. Sensors of this type are also called "laser gyros".
Such "laser gyros" are expensive. Laser gyros have a marked response threshold due to frequency coupling ("lock-in"). This response threshold has to be exceeded by a dither movement (Holzapfel "Optische Kreiselsensoren", VDI-Bericht No. 509, 1984).
Sensors which are known, for example, from European Pat. No. 0,290,723, use passive ring resonators. Passive ring resonators, again, have a closed optical path. Light from a laser is coupled into this optical path through couplers, once clockwise and once counter-clockwise. In this case, the ring resonator is not part of a laser, as in a laser gyro, but is an independent optical resonator. A well-defined portion of the light travelling clockwise in the ring resonator is coupled out through a coupler or a partially transparent mirror and is directed onto a first photoelectric detector. Also a well-defined portion of the light travelling counterclockwise in the ring resonator is coupled out and is directed onto a second photoelectric detector. The signals of the two photoelectric detectors depend on where the frequency of the lase light is located with respect to the resonance frequencies of the ring resonator existing for the two directions of propagation. When the ring resonator is ted to an angular rate about an axis normal to the plane of the ring resonator, the two resonance frequencies are changed in opposite sense. The electrical signals from the detectors are used to generate, by signal processing, output signals which are indicative of the angular rate of the ring resonator about an axis normal to the plane of the ring resonator. Realizations of this basic concept are known from, for example, European Pat. No. 0,290,723 which is cognate with U.S. Pat. No. 5,004,342, granted Apr. 2, 1991.
The prior art angular rate sensors using passive ring resonators usually require optical insulation of the laser, serving as light source, from the ring resonator by an "optical diode". Otherwise, the frequency of the laser will be influenced by the ring resonator. Such optical insulation is very expensive. If no such insulation is provided, as in the sensor according to European Pat. No. 0,290,723, the frequency of the laser has to be controlled through the current with strong feedback. This adversely affects the sensitivity of the sensor.